Several throttle gates for the dissipation of water energy or for throttling the flow of water conduits, dam-discharge openings or tubine units are known, which are of the ring, conical or similar type also known to provide flat throttling gates for this purpose as well. It has been found, however, that none of the these known types of throttling gates is suitable to be built into the draft tube of a bulb turbine. The reason therefor is to be found in their constructive characteristics and operational properties, so that at the tailwater side of the runner of the hydroelectric, where the kinetic energy of water is to be recovered, only a flat gate comes into consideration. Unfortunately, a simple flat throttling gate in the draft tubes has certain disadvantages. It is known from experimentally obtained data that the guide slots necessary for the guidance of the gate body cause additional energy losses by vortices which by some percents reduce the efficiency of the turbine, having the draft tube with the incorporated gate. The losses are especially intensive for high specific speed water turbines and at high discharges. Roughly estimated, the losses depend on the square of the absolute velocity of water.
In practice, there have been some attempts at eliminating the influence of the slots in the draft tubes, by the use of masks have been applied, connected either to the upper or to the lower part of the gate body and which block the slots when the gate is lifted. The disadvantage of these devices is the need for side openings, which are liable to be filled with sediment (sand and impurities). The use of the application of masks which are not connected to the body itself and are only inserted into the slots eliminates the operational readiness of the gate when a replacement of the mask by the body of the throttling gate is needed.
None of the cited solutions makes possible a secure operation of the gate as releasing device, because the operation of the gate body must be dynamic, i.e. its position should be adaptable to the water discharge desired rate of throttling.